P1006: NEXT GENERATION SEQUENCING (NGS) IN PEDIATRIC MASTOCYTOSIS
Authors: A. Angi, S. Bianchi, G. Palumbo, V. Filipponi, M. Rousseau, M. L. Moleti, F. Giona
Journal: HemaSphere (2022)

Background: Mastocytosis is a clonal disorder characterized by the accumulation of mast cells in the skin and, less frequently, in other organs. The disease is limited to the skin in children and may occur in one of three cutaneous variants: maculopapular cutaneous mastocytosis (MPCM), diffuse cutaneous mastocytosis (DCM) and mastocytoma. Systemic mastocytosis (SM) typically occurs in adults with KIT D816V mutation (3). Additional genetic mutations (TET2, N-RAS, SF3B1, ASXL1, etc.) have been detected using NGS in adults with mastocytosis (4). Currently, there is no data concerning the use of NGS in pediatric mastocytosis. Aims: The aims of the monocentric study, named MAS_PED1, that was approved by Ethics Committee, are to: a) identify patients (pts) with different cutaneous forms at risk of developing aggressive systemic disease; b) detect the known KIT D816V mutation using RT-PCR; c) identify different KIT, and/or additional gene mutations using NGS; and d) evaluate potential role of different mutations in the outcome of pts with mastocytosis, aged < 18 years at lesion onset. Methods: Pts with pediatric onset of mastocytosis, followed at the study Center were included. Peripheral blood samples were collected to detect KITD816V mutation, using both RT-PCR and ddPCR techniques, and to investigate other molecular mutations using NGS panels for rare and myeloid genes. Results: All 36 pts included in this study, 19 male and 17 female, with median age of lesion onset ranged from birth to 17.81 years (median 4.74 months), had a cutaneous mastocytosis (CM). Twenty-one of the 36 pts (58%) underwent cutaneous biopsy after a median from lesion onset of 3.77 months (range: 2.49 months – 11.6 years). According to the type of cutaneous lesions at diagnosis, pts were classified as: 20 (55%) MPCM, 10 (28%) DCM and 6 (17%) mastocytoma. The median tryptase value at the onset was 5 ng/ml, higher in MPCM (range: 1.2 - 141 ng/ml), than in DCM (range: 2.71 - 19.4 ng/ml), and in mastocytoma (range: 3.8 - 7.3 ng/ml). Two of the MPCM pts developed indolent SM (ISM) after 10 and 20 years from the onset disease. RT-PCR and NGS assessment were performed in 17 pts (47%), 15 CM and 2 ISM. RT-PCR identified KIT D816V mutation in 4 pts (2 MPCM, 1 DCM, 1 ISM) while NGS revealed KIT D816V in 3 pts and other KIT mutations, KIT D816Y and KIT Y553C, in 2 pts. An additional 10 myeloid gene mutations were detected by NGS technique: 5 were already known (ASXL1 G1397S; JAK2 L393V; KITD816Y; LNK E208Q; TET2 Y867H) while the other 5 have not been previously described (ETV6A215P; KIT Y553C; NFE2 1291T; SH2B3 G382D; SH2B3 L438V). Mutations in myeloid genes were prevalent in the signalling functional group. A single mutation was found in 7 pts (3MPCM, 3 DCM, 1ISM), while two or more mutations were found in 3 DCM pts, 2 of them had a spontaneous regression. Two different mutations (KIT Y553C and SH2B3 L438V) were found in two sisters. Overall, 9/36 pts (5 DCM, 3 MPCM, 1 mastocytoma) presented spontaneous complete regression of cutaneous lesions after a median time of 25 months (range: 17 months - 25 years). Summary/Conclusion: KIT mutations resulted in 35% of the children tested. The RT-PCR technique resulted more sensitive in finding KIT D816V, while NGS in detecting other mutations. ASXL1, JAK2 and TET2 mutations, detected in our population, are different to those reported in adult pts with SM. The presence of multiple mutations in the DCM form, does not appear to influence the evolution to a systemic form, unlike in adults.